The stereochemistry of the two elimination steps in the biosynthesis of the gibberellin precursor kaurene is to be clarified by experiments with labelled substrates using soluble enzyme preparations. The elucidation of the hidden stereochemistry associated with the biosynthesis of a family of related diterpenes is proposed. The structural and biosynthetic stereochemistry of casbene, a cyclopropane-containing phytoalexin in castor beans, constitutes another aspect of the project. The substrate specificity of the cyclases is to be explored by use of a series of analogues of geranylgeranyl and copalyl pyrophosphates. A second major objective is the determination of the stereochemistry which attends the enzymatic oxidations at the C-4 methyl group of kaurene en route to the gibberellins and at the C-4 and C-14 methyl groups of sterols on the biosynthetic pathway to cholesterol. A combination of chemical and spectroscopic procedures is outlined for the synthesis and stereochemical assignment of the prochiral hydrogens of hydroxymethyl groups. Carbon-13 labelling and NMR is to be utilized to ascertain the biosynthetic route to the novel sesterpene alcohol moenocinol, a component of the antibotic moenomycin. The use of multiply labelled isoepentenyl pyrophosphate and delta 8-cholesten-3beta-ol is proposed to determine whether a hydroen isotope effect exists in these enzymatic isomerization reactions.